A flight sim is a waste of time for renewing an SEP rating. Go to the CAA site and download the SRG1157 form that we use to record the flight test. That will show you the skills you will cover.
You won't be able to practice, steep turns, PFL's, EFATO's etc on a SIM.
I do a lot of skill tests for people with expired licences and ratings and generally find an hours brush up before the test is enough. I had one guy who had not flown for 12 years and only took a couple of hours. I also regularly get people who have not flown for a couple of years jump straight into the test and pass without problems.
You are young and unless you are a muppet should take little to get back up to speed.